1. Field of the Invention
The invention relates to a method for recognizing fires according to the scattered light principle by pulsed emission of a radiation of a first wavelength along a first radiation axis as well as a radiation of a second wavelength which is shorter than the first wavelength along a second radiation axis into a measuring volume and by measuring the radiation scattered on the particles located in the measuring volume under a forward scattering angle of more than 90° and under a backward scattering angle of less than 90°. The invention further relates to a scattered-light fire detector for performing this method.
2. Description of the Related Art
A scattered-light detector is known from WO 01/59 737 which is provided especially for installation in ventilation and air-conditioning conduits, which operates according to the aforementioned method and where a first light-emitting diode (LED) emits infrared light and a second LED emits blue light into its measuring chamber. The LEDs are pulsed in an alternating fashion. The radiation produced by the “infrared” LED allows recognizing large particles which are typical for a smouldering fire. The scattered radiation produced by the “blue” LED allows recognizing small particles which are typical for fires with open flames. This is explained by Rayleigh's law, according to which the intensity of the scattered light decreases with the fourth power of the wavelength for particles which are smaller than the wavelength. Although the latter is correct, it does not fulfill the actual conditions in recognizing fires according to the scattered light principle. The known fire detector comprises only a single photodetector which supplies only two pieces of information on the scattered light intensities, namely, depending on the embodiment, either the intensity of the forward scattered radiation in the infrared and in the blue wavelength region or the respective intensities of the backward scattered radiations or also the intensity of the forward scattered radiation in the infrared wavelength region and the backward scattered radiation in the blue wavelength region. The respective arrangement criteria lead to the consequence, however, that the measuring volumes from which the respective scattered radiation is obtained are not identical.
From DE 199 02 319, a fire detection method is known in which the alarm decision is made depending on the ratio of the intensity of the IR forward scattered radiation to the intensity of the IR backward scattered radiation. The respective fire detector works optionally with two infrared LEDs and a photodetector or vice-versa with one infrared LED and two photodetectors. The angle under which the forward scattered radiation is measured is preferably 140°, and the angle under which the backward scattered radiation is measured is preferably 70°. The formation of the ratio of the intensities of the forward and backward scattered radiation allows distinguishing bright from dark types of smoke, because bright smoke supplies a high forward scattered signal and a comparatively small backward scattered signal, whereas, conversely, dark smoke supplies a lower forward scattered signal and a comparatively high backward scattered signal. The processing of the absolute intensities or signal level by taking into account the principally lower intensities in the backward scattering region in relationship to the intensities produced in the forward scattering region by the same particles with the same intensity and the simultaneous processing of the ratios or quotients of these signals also allow distinguishing certain deceptive values of smoke. For example, water vapor in high concentration produces a high forward scattered signal which according to the older state of the art leads to the initiation of an alarm, in this case, to a false alarm. The formation of the quotient from the forward scattered intensity and the backward scattered intensity leads to a value which is characteristic for water vapor, which value is substantially independent of the concentration. By determining this quotient and considering it in the further signal processing it is thus possible to suppress any false alarms that would occur otherwise. The known method and the detector which operates according to this method have a common feature with all other known constructions of scattered-light fire detectors which operate on the basis of infrared light, which feature is the disadvantage of an inadequate sensitivity for small and very small particles. This makes it more difficult to recognize open fires in due time, and especially wood fires whose smoke is characterized by a very small particle size. In the case of a respective hazardous situation it is therefore still necessary to use ionization fire detectors which respond very well to small particles and which work with a preparation of low radioactivity. Due to this radioactive preparation, the production of ionization fire detectors is complex and their use is unpopular and even generally prohibited in a number of countries.